Traveling patterns in seagrass meadows

What is it about?

A new study has found that strips of vegetation that form in seagrass meadows such as Posidonia oceanica move at a constant speed and can collide with each other in a process of annihilation. The authors propose a model that reproduces these dynamics and at the same time allows to check the state of the meadows.

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Photo by Benjamin L. Jones on Unsplash

Photo by Benjamin L. Jones on Unsplash

Why is it important?

Posidonia meadows are an important source of ecosystem services and act as carbon sinks in coastal regions around the world. However, these seagrass meadows are known to be under threat due to multiple anthropogenic pressures, leading to increased seagrass mortality. In general, when reproduction and mortality rates are close to equilibrium, scale-dependent feedbacks are the dynamics that govern the spatio-temporal evolution of seagrass meadows. These interactions between plants can generate regular patterns such as those observed in the fairy circles in Namibia or the labyrinths of the Negev desert, so studying these patterns and their evolution is key to diagnosing the health of vegetation expanses. We have discovered that this situation of high mortality leads in some cases to the formation of traveling pulses of vegetation, strips of Posidonia in the specific case of Mediterranean meadows, approximately 1.5 m wide that advance without changing shape at a speed of a few centimeters per year, and that generate complex spatio-temporal patterns in the form of rings, spirals or arcs (Fig. 1). These structures arise due to high plant mortality caused by the absorption of sulfur by the roots. This sulfide comes from the decomposition of organic matter by bacteria in the absence of oxygen. The resulting spatiotemporal patterns resemble those formed in other excitable media, such as cardiac tissue or the Belousov-Zhabotinsky reaction, but on a much larger scale. The researchers have developed a mathematical model that reproduces the observed seascapes and predicts the annihilation of these circular structures when they collide with each other, a hallmark of excitable pulses. They have also shown that field images and radial profiles of vegetation, as well as the concentration of sulfide in the sediment, are consistent with the predictions of the theoretical model. The simulations reproduce remarkably well the evolution of the rings from 1973 to the present, including the self-destruction of two vegetation strips upon collision.